1. Field of the Invention
The present invention relates to selective combining of modulated signals.
2. Description of the Prior Art
FIG. 1 illustrates a prior art combiner 10 which has been used in base stations to selectively provide an output of a single modulated carrier of higher power output or an output comprising two distinguishable carriers of lower power from a pair of quadrature modulators 12 and 14. The quadrature modulators 12 and 14 provide respectively combined output signals 16 and 18 from summers 17 and 19 respectively as inputs to a switch based signal router 20′. The signal router 20′ selectively permits the choosing of the combined output signals 16 and 18 being applied independently to power amplifiers 20 and 22 which are input to a quadrature splitter 24 or output signal 16 applied to quadrature splitter 26 and split into quadrature components and then applied to power amplifiers 20 and 22. The quadrature splitter 24 sums the outputs of power amplifiers 20 and 22. The position of switch 40 determines if the signals that arrive at quadrature splitter 24 are coherent or incoherent. As is known, a coherent output has a carrier therein derived from a single frequency source and an incoherent output has the carriers therein derived from unrelated frequency sources.
For incoherent operation, the switches 40, 42, 44 and 46 of the signal router 20′ are switched to the upper position which provides a path of the signals 16 and 18 into the power amplifiers 20 and 22 so the signals are amplified and input to the quadrature splitter 24. The amplifier signal from power amplifier 20 applied to the 0° input of the quadrature splitter 24 is not phase shifted in the combined output of the quadrature splitter and the amplifier signal from the power amplifier 22 applied to the 90° input of the quadrature splitter is phase shifted approximately 90° in the combined output. The result is the combination of the two independent carriers with the two carriers still being completely distinguishable. Because the total combined power will need to remain the same, the power of the individual components is reduced.
For coherent operation the external switches 40, 42, 44 and 46 of the signal router 20′ are switched to the lower position which grounds the output from the summer 19. The input signal 16 is applied to a quadrature splitter 26 which provides a 0° and 90° output. The output from the 90° input has a 90° phase shift relative to the 0° input and is applied to the power amplifier 20. The output from the 0° input is applied without a phase shift to the power amplifier 22. The outputs from the power amplifiers 20 and 22 are input to the 90° and 0° inputs of the quadrature modulator 24. A phase shift of approximately 90° is introduced in the combined output from the 90° input of the quadrature splitter 24 and substantially no phase shift is introduced in the combined output from the 0° input. The output from the quadrature modulator 24 is a single modulated carrier of higher power and has a combined amplified power equal to the sum of the power produced by amplifiers 20 and 22. An envelope detector 28 detects an envelope of the combined output for conventional monitoring of the combined output combiner.
The quadrature splitters 24 and 26 have a voltage transfer function as follows:             hybrid_      ⁢      90        ⁢                   :=          [                                                                  1                2                            ⁢                              2                                                                                                        1                2                            ⁢              j              ⁢                              2                                                        ]                  hybrid_      ⁢      90        ⁢                   :=          [                                                                  1                2                            ⁢                              2                                                                                                        1                2                            ⁢              j              ⁢                              2                                                        ]      
The quadrature modulators 12 and 14 operate as follows: The input signals I1, Q1 and I2, Q2 are respectively applied to different modulators 30. The quadrature modulators 12 and 14 shift the signals from the baseband input to a carrier which is modulated by the input signals. The incoherent carrier signals F1 and F2 are provided by signal sources 32 and 34 which may be analog sinusoidal carriers or digital representations produced in a digital signal processor. The quadrature splitters 36 respectively provide an unshifted carrier and a 90° shifted carrier output to the modulators 30 which also have inputs of the aforementioned I and Q signals. As is understood, the quadrature modulators 12 and 14 each output modulated signals which are complex signals containing orthogonal I and Q components. If the modulated carriers are in the analog domain, the representation thereof may be by sine and cosine functions as discussed below.
While the quadrature modulators 12 and 14 have been described with an architecture for providing modulated analog signals, it should be understood that digital signal processors as quadrature modulators may be used.
The prior art has the disadvantage of requiring the quadrature splitter 26 and four switches 40-46 in order to obtain the requisite alignment of phase and frequency for both incoherent and coherent combining. This additional hardware adds cost, complexity, increases size and introduces power loss resultant from the physical components identified above and is external to the quadrature modulators.